Earth-sized planets that host life should be far easier to find around parent stars that are white dwarfs, the ultimate incarnations of stars like the sun, a new study shows.
White dwarfs are the dense stellar cores that remain after a sun-like star runs out of fuel and goes through its expanding, red giant phase, a process that will consume its inner planets. In our solar system, for example, Mercury, Venus and possibly Earth will be destroyed when the sun evolves into a red giant some 4.5 billion years from now.
But the system won't necessarily be doomed.
Outer planets may migrate inward, closer to the star, and new worlds may form. Not all will be in stable orbits, but an Earth-sized world located about 1 million miles away from a host white dwarf star would have a temperature roughly the same as Earth’s. At that distance, the planet could have liquid water on its surface, a condition believed to be necessary for life.
Scientists are developing techniques to scan the atmospheres of planets beyond the solar system for oxygen and other chemical signs of life. It's a laborious and time-consuming process to separating out light passing through a planet’s atmosphere from all the background starlight.
But Earth-sized planets circling white dwarf stars, which are themselves about as big as Earth, make for much bigger needles in extrasolar planet haystacks.
Avi Loeb, a theorist at the Harvard-Smithsonian Center for Astrophysics, figures the upcoming James Webb Space Telescope, a successor to the Hubble observatory, would need only about five hours of observing time to look for biomarkers in the atmosphere of a planet circling in a white dwarf’s habitable zone.
“Usually the background star is so much brighter, it’s so much bigger than the planet that absorption (of light) due to the atmosphere is a very small signal that you have to fish out of the much more prominent emission from the background star,” Loeb told Discovery News.
“In the case of the white dwarf, it’s sort of the best of all circumstances, where the object that is blocking the star is of the same size as the star itself. That offers the best prospect for detecting the absorption due to the atmosphere, relative to the background light,” he said.